GENESIS OF THE ALKALINE ROCKS 127) 
Loct of magmatic assimilation.—Again, the importance of batho- 
lithic assimilation is to be properly gauged only if all its different 
loci are kept in mind. In part it is marginal, affecting the roof 
region and also the vast walls, independently hot, at great depth. 
For the rest it consists of the fusion or solution of xenoliths, either 
near the roof or at deep levels. Accordingly batholithic assimila- 
tion may be described as: 
1. Marginal: (a) High-level, (6) Abyssal. 
2. Xenolithic: (a) High-level, (6) Abyssal. 
Of the four kinds the xenolithic-abyssal and the marginal- 
abyssal are probably most important. Yet precisely these loci 
of the geological work, typically very deep, can seldom or never be 
exposed to view by erosion. Hence batholithic syntexis, beside 
which all other is an almost vanishing quantity, can be inferred 
only from its results. Its whole meaning for petrology is not to be 
directly deduced from visible contacts. 
Bowen’s special suggestions as to the origin of alkaline types.— 
The foregoing considerations are vital in the theory of alkaline 
rocks, but Bowen’s petrogenic scheme raises doubts when some of 
_ his more specific points are examined. He recognizes two principal 
modes of origin for the alkaline rocks, as stated in the following 
quotations from his memoir (pp. 56, 57): 
It has been shown that at the biotite granite stage [of evolving basaltic 
magma], and to a lesser extent in preceding stages, reactions take place in the 
liquid whose principal feature is the breakdown of polysilicate molecules, 
probably under the influence of water, to the simpler orthosilicate molecules, 
among them KAISiO, and NaAlSiO,. The precipitation of KAISi;03, NaASi;Os, 
KAISiO, in mica, and SiO, as quartz means the concentration in the liquid of 
all the other molecules indicated in the reactions given on pp. 44 and 45. These 
are principally NaAISiO, and the volatile constituents, water, chlorine, etc., 
with their compounds. If the crystals of the biotite granite stage, including 
quartz, sink out of this liquid, then the concentration of NaAISiO, will finally 
reach a stage where nephelite will begin to precipitate. There may also result 
a concentration of CO2, S, SO;, Cl, etc., sufficient to cause their precipitation in 
compounds such as cancrinite, lazurite, hauynite, and sodalite, minerals which 
are peculiar to nephelite syenites and related rocks. ... . 
Differentiation during crystallization from these very fluid [alkaline] 
magmas will take place very freely, and the formation of both highly femic and 
highly alkalic types (rich in feldspathoids) may result. The more “basic” 
